Rescheduling a failed memory request in a processor

ABSTRACT

Devices and techniques for rescheduling a failed memory request in a processor are described herein. When a memory request for a thread is denied at a point in the execution pipeline of the processor beyond a thread rescheduling point, the thread can be placed into a memory response path of the processor. An indicator that a register write-back will not occur for the thread can also be provided. Then, the thread can be rescheduled with other threads in the memory response path.

BACKGROUND

Chiplets are an emerging technique for integrating various processing functionalities. Generally, a chiplet system is made up of discrete modules (each a “chiplet”) that are integrated on an interposer, and in many examples interconnected as desired through one or more established networks, to provide a system with the desired functionality. The interposer and included chiplets can be packaged together to facilitate interconnection with other components of a larger system. Each chiplet can include one or more individual integrated circuits (ICs), or “chips”, potentially in combination with discrete circuit components, and commonly coupled to a respective substrate to facilitate attachment to the interposer. Most or all chiplets in a system will be individually configured for communication through the one or more established networks.

The configuration of chiplets as individual modules of a system is distinct from such a system being implemented on single chips that contain distinct device blocks (e.g., intellectual property (IP) blocks) on one substrate (e.g., single die), such as a system-on-a-chip (SoC), or multiple discrete packaged devices integrated on a printed circuit board (PCB). In general, chiplets provide better performance (e.g., lower power consumption, reduced latency, etc.) than discrete packaged devices, and chiplets provide greater production benefits than single die chips. These production benefits can include higher yields or reduced development costs and time.

Chiplet systems can include, for example, one or more application (or processor) chiplets and one or more support chiplets. Here, the distinction between application and support chiplets is simply a reference to the likely design scenarios for the chiplet system. Thus, for example, a synthetic vision chiplet system can include, by way of example only, an application chiplet to produce the synthetic vision output along with support chiplets, such as a memory controller chiplet, a sensor interface chiplet, or a communication chiplet. In a typical use case, the synthetic vision designer can design the application chiplet and source the support chiplets from other parties. Thus, the design expenditure (e.g., in terms of time or complexity) is reduced because by avoiding the design and production of functionality embodied in the support chiplets. Chiplets also support the tight integration of IP blocks that can otherwise be difficult, such as those manufactured using different processing technologies or using different feature sizes (or utilizing different contact technologies or spacings). Thus, multiple IC's or IC assemblies, with different physical, electrical, or communication characteristics can be assembled in a modular manner to provide an assembly providing desired functionalities. Chiplet systems can also facilitate adaptation to suit needs of different larger systems into which the chiplet system will be incorporated. In an example, IC's or other assemblies can be optimized for the power, speed, or heat generation for a specific function—as can happen with sensors—can be integrated with other devices more easily than attempting to do so on a single die. Additionally, by reducing the overall size of the die, the yield for chiplets tends to be higher than that of more complex single die devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the disclosure. The drawings, however, should not be taken to limit the disclosure to the specific embodiments, but are for explanation and understanding only.

FIGS. 1A and B illustrate an example of a chiplet system, according to an embodiment.

FIG. 2 illustrates components of an example of a memory controller chiplet, according to an embodiment.

FIG. 3 illustrates components in an example of a programmable atomic unit (PAU), according to an embodiment.

FIG. 4 illustrates a processing flow through processor components to reschedule a failed memory request, according to an embodiment.

FIG. 5 is a flow chart of an example of a method for rescheduling a failed memory request in a processor, according to an embodiment.

FIG. 6 is a block diagram of an example of a machine with which, in which, or by which embodiments of the present disclosure can operate.

DETAILED DESCRIPTION

FIG., described below, offers an example of a chiplet system and the components operating therein. The illustrated chiplet system includes a memory controller. This memory controller includes a programmable atomic unit (PAU) to execute a custom program, a programmable atomic operator in response to a memory request for the programmable atomic operator. Additional details about the PAU are described below with respect to FIGS. and 3. The processor of the PAU can be barrel-multithreaded and pipelined. An issue can arise with such a processor executing a programmable atomic operator that involves detecting a problem with making a memory request by a thread. Traditionally, such detection happens when the memory request is being made, which is often a late stage in the pipeline.

If a thread attempts to issue a memory request and detects that the request will fail (e.g., the thread reads hazard indications of an outstanding memory request or an inconsistent memory state) past existing thread rescheduling paths, the compute pipeline will stall to allow relevant hazards to clear prior to resuming execution. The resulting stall places burdens on each logic stage of the pipeline to tolerate (e.g., handle or account for) such a stall, generally resulting in additional circuitry that would otherwise be unnecessary. Moreover, the thread scheduler will require additional circuitry to detect the clearance of the hazards to reschedule the thread. In addition to the additional circuitry in the pipeline stages and the scheduler, there is performance cost in addressing the stall. In a barrel-multithreaded processor, other allocated threads also suffer a performance cost for the failed memory request.

To address the failed memory request issue, threads that are denied issuing a memory request are rescheduled as if the memory request was successful via the traditional memory response path. This rescheduling can be modified by path (e.g., placement of the failed memory request in an interim queue) and type (e.g., one or more indications are placed in the thread rescheduling to indicate that no data from memory will be returned). Thus, if a problem arises with issuing the memory request during execution of the pipeline past normal thread rescheduling points, the thread halts execution and is placed in a queue to be pulled into the traditional memory response pipeline. The indications (e.g., bits) are set to denote this special case does not imply a register writeback, and the thread is pushed into the general thread scheduler queue by way of the memory response path.

This technique greatly simplifies cases where threads in a barrel-multithreaded processor are unable to issue memory requests late in the pipeline. Stalling to wait for the hazard to clear would add latency to all allocated threads and a logic cost (e.g., additional circuitry) to all stages of the pipeline to handle the stall. By enabling the thread to halt and reschedule via the existing successful memory response path, hazards can be cleared without costing any additional cycles and the thread is rescheduled with no penalty to other threads. Additional details and examples are provided below.

FIGS. A and B illustrate an example of a chiplet system 10, according to an embodiment. FIG. A is a representation of the chiplet system 10 mounted on a peripheral board 05, that can be connected to a broader computer system by a peripheral component interconnect express (PCIe), for example. The chiplet system 10 includes a package substrate 15, an interposer 20, and four chiplets, an application chiplet 25, a host interface chiplet 35, a memory controller chiplet 40, and a memory device chiplet 50. Other systems can include many additional chiplets to provide additional functionalities as will be apparent from the following discussion. The package of the chiplet system 10 is illustrated with a lid or cover 65, though other packaging techniques and structures for the chiplet system can be used. FIG. B is a block diagram labeling the components in the chiplet system for clarity.

The application chiplet 25 is illustrated as including a network-on-chip (NOC) 30 to support a chiplet network 55 for inter-chiplet communications. In example embodiments NOC 30 can be included on the application chiplet 25. In an example, NOC 130 can be defined in response to selected support chiplets (e.g., chiplets 35, 40, and 50) thus enabling a designer to select an appropriate number or chiplet network connections or switches for the NOC 30. In an example, the NOC 30 can be located on a separate chiplet, or even within the interposer 20. In examples as discussed herein, the NOC 30 implements a chiplet protocol interface (CPI) network.

The CPI is a packet-based network that supports virtual channels to enable a flexible and high-speed interaction between chiplets. CPI enables bridging from intra-chiplet networks to the chiplet network 55. For example, the Advanced eXtensible Interface (AXI) is a widely used specification to design intra-chip communications. AXI specifications, however, cover a great variety of physical design options, such as the number of physical channels, signal timing, power, etc. Within a single chip, these options are generally selected to meet design goals, such as power consumption, speed, etc. However, to achieve the flexibility of the chiplet system, an adapter, such as CPI, is used to interface between the various AXI design options that can be implemented in the various chiplets. By enabling a physical channel to virtual channel mapping and encapsulating time-based signaling with a packetized protocol, CPI bridges intra-chiplet networks across the chiplet network 55.

CPI can use a variety of different physical layers to transmit packets. The physical layer can include simple conductive connections, or can include drivers to increase the voltage, or otherwise facilitate transmitting the signals over longer distances. An example of one such physical layer can include the Advanced Interface Bus (AIB), which in various examples, can be implemented in the interposer 20. AIB transmits and receives data using source synchronous data transfers with a forwarded clock. Packets are transferred across the AlB at single data rate (SDR) or dual data rate (DDR) with respect to the transmitted clock. Various channel widths are supported by AIB. AIB channel widths are in multiples of 20 bits when operated in SDR mode (20, 40, 60, . . . ), and multiples of 40 bits for DDR mode: (40, 80, 120, . . . ). The AIB channel width includes both transmit and receive signals. The channel can be configured to have a symmetrical number of transmit (TX) and receive (RX) input/outputs (I/Os), or have a non-symmetrical number of transmitters and receivers (e.g., either all transmitters or all receivers). The channel can act as an AIB principal or subordinate depending on which chiplet provides the principal clock. AIB I/O cells support three clocking modes: asynchronous (i.e. non-clocked), SDR and DDR. In various examples, the non-clocked mode is used for clocks and some control signals. The SDR mode can use dedicated SDR only I/O cells, or dual use SDR/DDR I/O cells.

In an example, CPI packet protocols (e.g., point-to-point or routable) can use symmetrical receive and transmit I/O cells within an AIB channel. The CPI streaming protocol allows more flexible use of the AIB I/O cells. In an example, an AIB channel for streaming mode can configure the I/O cells as all TX, all RX, or half TX and half RX. CPI packet protocols can use an AIB channel in either SDR or DDR operation modes. In an example, the AlB channel is configured in increments of 80 I/O cells (i.e. 40 TX and 40 RX) for SDR mode and 40 I/O cells for DDR mode. The CPI streaming protocol can use an AIB channel in either SDR or DDR operation modes. Here, in an example, the AIB channel is in increments of 40 I/O cells for both SDR and DDR modes. In an example, each AIB channel is assigned a unique interface identifier. The identifier is used during CPI reset and initialization to determine paired AIB channels across adjacent chiplets. In an example, the interface identifier is a 20-bit value comprising a seven-bit chiplet identifier, a seven-bit column identifier, and a six-bit link identifier. The AIB physical layer transmits the interface identifier using an AIB out-of-band shift register. The 20-bit interface identifier is transferred in both directions across an AlB interface using bits 32-51 of the shift registers.

AIB defines a stacked set of AIB channels as an AIB channel column. An AlB channel column has some number of AIB channels, plus an auxiliary channel. The auxiliary channel contains signals used for AIB initialization. All AIB channels (other than the auxiliary channel) within a column are of the same configuration (e.g., all TX, all RX, or half TX and half RX, as well as having the same number of data I/O signals). In an example, AIB channels are numbered in continuous increasing order starting with the AIB channel adjacent to the AUX channel. The AIB channel adjacent to the AUX is defined to be AlB channel zero.

Generally, CPI interfaces on individual chiplets can include serialization-deserialization (SERDES) hardware. SERDES interconnects work well for scenarios in which high-speed signaling with low signal count are desirable. SERDES, however, can result in additional power consumption and longer latencies for multiplexing and demultiplexing, error detection or correction (e.g., using block level cyclic redundancy checking (CRC)), link-level retry, or forward error correction. However, when low latency or energy consumption is a primary concern for ultra-short reach, chiplet-to-chiplet interconnects, a parallel interface with clock rates that allow data transfer with minimal latency can be utilized. CPI includes elements to minimize both latency and energy consumption in these ultra-short reach chiplet interconnects.

For flow control, CPI employs a credit-based technique. A recipient, such as the application chiplet 25, provides a sender, such as the memory controller chiplet 40, with credits that represent available buffers. In an example, a CPI recipient includes a buffer for each virtual channel for a given time-unit of transmission. Thus, if the CPI recipient supports five messages in time and a single virtual channel, the recipient has five buffers arranged in five rows (e.g., one row for each unit time). If four virtual channels are supported, then the recipient has twenty buffers arranged in five rows. Each buffer holds the payload of one CPI packet.

When the sender transmits to the recipient, the sender decrements the available credits based on the transmission. Once all credits for the recipient are consumed, the sender stops sending packets to the recipient. This ensures that the recipient always has an available buffer to store the transmission.

As the recipient processes received packets and frees buffers, the recipient communicates the available buffer space back to the sender. This credit return can then be used by the sender allow transmitting of additional information.

Also illustrated is a chiplet mesh network 60 that uses a direct, chiplet-to-chiplet technique without the need for the NOC 30. The chiplet mesh network 60 can be implemented in CPI, or another chiplet-to-chiplet protocol. The chiplet mesh network 60 generally enables a pipeline of chiplets where one chiplet serves as the interface to the pipeline while other chiplets in the pipeline interface only with themselves.

Additionally, dedicated device interfaces, such as one or more industry standard memory interfaces 145 (such as, for example, synchronous memory interfaces, such as DDRS, DDR 6), can also be used to interconnect chiplets. Connection of a chiplet system or individual chiplets to external devices (such as a larger system can be through a desired interface (for example, a PCIE interface). Such as external interface can be implemented, in an example through a host interface chiplet 35, which in the depicted example, provides a PCIE interface external to chiplet system 10. Such dedicated interfaces 45 are generally employed when a convention or standard in the industry has converged on such an interface. The illustrated example of a Double Data Rate (DDR) interface 45 connecting the memory controller chiplet 40 to a dynamic random access memory (DRAM) memory device 50 is just such an industry convention.

Of the variety of possible support chiplets, the memory controller chiplet 40 is likely present in the chiplet system 10 due to the near omnipresent use of storage for computer processing as well as sophisticated state-of-the-art for memory devices. Thus, using memory device chiplets 50 and memory controller chiplets 40 produced by others gives chiplet system designers access to robust products by sophisticated producers. Generally, the memory controller chiplet 40 provides a memory device specific interface to read, write, or erase data. Often, the memory controller chiplet 40 can provide additional features, such as error detection, error correction, maintenance operations, or atomic operator execution. For some types of memory, maintenance operations tend to be specific to the memory device 50, such as garbage collection in NAND flash or storage class memories, temperature adjustments (e.g., cross temperature management) in NAND flash memories. In an example, the maintenance operations can include logical-to-physical (L2P) mapping or management to provide a level of indirection between the physical and logical representation of data. In other types of memory for example DRAM, some memory operations, such as refresh can be controlled by a host processor or of a memory controller at some times, and at other times controlled by the DRAM memory device, or by logic associated with one or more DRAM devices, such as an interface chip (in an example, a buffer).

Atomic operators are a data manipulation that, for example, can be performed by the memory controller chiplet 40. In other chiplet systems, the atomic operators can be performed by other chiplets. For example, an atomic operator of “increment” can be specified in a command by the application chiplet 25, the command including a memory address and possibly an increment value. Upon receiving the command, the memory controller chiplet 40 retrieves a number from the specified memory address, increments the number by the amount specified in the command, and stores the result. Upon a successful completion, the memory controller chiplet 40 provides an indication of the commands success to the application chiplet 25. Atomic operators avoid transmitting the data across the chiplet network 60, resulting in lower latency execution of such commands.

Atomic operators can be classified as built-in atomics or programmable (e.g., custom) atomics. Built-in atomics are a finite set of operations that are immutably implemented in hardware. Programmable atomics are small programs that can execute on a programmable atomic unit (PAU) (e.g., a custom atomic unit (CAU)) of the memory controller chiplet 40. FIG. illustrates an example of a memory controller chiplet that discusses a PAU.

The memory device chiplet 50 can be, or include any combination of, volatile memory devices or non-volatile memories. Examples of volatile memory devices include, but are not limited to, random access memory (RAM)—such as DRAM) synchronous DRAM (SDRAM), graphics double data rate type 6 SDRAM (GDDR6 SDRAM), among others. Examples of non-volatile memory devices include, but are not limited to, negative-and-(NAND)-type flash memory, storage class memory (e.g., phase-change memory or memristor based technologies), ferroelectric RAM (FeRAM), among others. The illustrated example includes the memory device 50 as a chiplet, however, the memory device 50 can reside elsewhere, such as in a different package on the peripheral board 05. For many applications, multiple memory device chiplets can be provided. In an example, these memory device chiplets can each implement one or multiple storage technologies. In an example, a memory chiplet can include, multiple stacked memory die of different technologies, for example one or more static random access memory (SRAM) devices stacked or otherwise in communication with one or more dynamic random access memory (DRAM) devices. Memory controller 140 can also serve to coordinate operations between multiple memory chiplets in chiplet system 110; for example to utilize one or more memory chiplets in one or more levels of cache storage, and to use one or more additional memory chiplets as main memory. Chiplet system 110 can also include multiple memory controllers 140, as can be used to provide memory control functionality for separate processors, sensors, networks, etc. A chiplet architecture, such as chiplet system 110 offers advantages in allowing adaptation to different memory storage technologies; and different memory interfaces, through updated chiplet configurations, without requiring redesign of the remainder of the system structure.

FIG. illustrates components of an example of a memory controller chiplet 05, according to an embodiment. The memory controller chiplet 05 includes a cache 10, a cache controller 15, an off-die memory controller 20 (e.g., to communicate with off-die memory 75), a network communication interface 25 (e.g., to interface with a chiplet network 85 and communicate with other chiplets), and a set of atomic and merge units 50. Members of this set can include, for example, a write merge unit 55, a memory hazard unit 60, built-in atomic unit 65, or a PAID 70. The various components are illustrated logically, and not as they necessarily would be implemented. For example, the built-in atomic unit 65 likely comprises different devices along a path to the off-die memory. For example, the built-in atomic unit 65 could be in an interface device/buffer on a memory chiplet, as discussed above. In contrast, the programmable atomic unit 70 could be implemented in a separate processor on the memory controller chiplet 05 (but in various examples can be implemented in other locations, for example on a memory chiplet).

The off-die memory controller 20 is directly coupled to the off-die memory 75 (e.g., via a bus or other communication connection) to provide write operations and read operations to and from the one or more off-die memory, such as off-die memory 75 and off-die memory 80. In the depicted example, the off-die memory controller 20 is also coupled for output to the atomic and merge unit 50, and for input to the cache controller 15 (e.g., a memory side cache controller).

In the example configuration, cache controller 15 is directly coupled to the cache 10, and can be coupled to the network communication interface 25 for input (such as incoming read or write requests), and coupled for output to the off-die memory controller 20.

The network communication interface 25 includes a packet decoder 30, network input queues 35, a packet encoder 40, and network output queues 45 to support a packet-based chiplet network 85, such as CPI. The chiplet network 85 can provide packet routing between and among processors, memory controllers, hybrid threading processors configurable processing circuits, or communication interfaces. In such a packet-based communication system, each packet typically includes destination and source addressing, along with any data payload or instruction. In an example, the chiplet network 85 can be implemented as a collection of crossbar switches having a folded Clos configuration, or a mesh network providing for additional connections, depending upon the configuration.

In various examples, the chiplet network 85 can be part of an asynchronous switching fabric. Here, a data packet can be routed along any of various paths, such that the arrival of any selected data packet at an addressed destination can occur at any of multiple different times, depending upon the routing. Additionally, chiplet network 85 can be implemented at least in part as a synchronous communication network, such as a synchronous mesh communication network. Both configurations of communication networks are contemplated for use for examples in accordance with the present disclosure.

The memory controller chiplet 05 can receive a packet having, for example, a source address, a read request, and a physical address. In response, the off-die memory controller 20 or the cache controller 15 will read the data from the specified physical address (which can be in the off-die memory 75 or in the cache 10), and assemble a response packet to the source address containing the requested data. Similarly, the memory controller chiplet 05 can receive a packet having a source address, a write request, and a physical address. In response, the memory controller chiplet 05 will write the data to the specified physical address (which can be in the cache 10 or in the off-die memories 75 or 80), and assemble a response packet to the source address containing an acknowledgement that the data was stored to a memory.

Thus, the memory controller chiplet 05 can receive read and write requests via the chiplet network 85 and process the requests using the cache controller 15 interfacing with the cache 10, if possible. If the request cannot be handled by the cache controller 15, the off-die memory controller 20 handles the request by communication with the off-die memories 75 or 80, the atomic and merge unit 50, or both. As noted above, one or more levels of cache can also be implemented in off-die memories 275 or 280; and in some such examples can be accessed directly by cache controller 215. Data read by the off-die memory controller 20 can be cached in the cache 10 by the cache controller 15 for later use.

The atomic and merge unit 250 are coupled to receive (as input) the output of the off-die memory controller 220, and to provide output to the cache 210, the network communication interface 225, or directly to the chiplet network 285. The memory hazard unit 60, write merge unit 55 and the built-in (e.g., predetermined) atomic unit 65 can each be implemented as state machines with other combinational logic circuitry (such as adders, shifters, comparators, AND gates, OR gates, XOR gates, or any suitable combination thereof) or other logic circuitry. These components can also include one or more registers or buffers to store operand or other data. The PAU 70 can be implemented as one or more processor cores or control circuitry, and various state machines with other combinational logic circuitry or other logic circuitry, and can also include one or more registers buffers or memories to store addresses, executable instructions, operand and other data or can be implemented as a processor.

The write merge unit 55 receives read data and request data, and merges the request data and read data to create a single unit having the read data and the source address to be used in the response or return data packet). The write merge unit 55 provides the merged data to the write port of the cache 10 (or, equivalently, to the cache controller 15 to write to the cache 10). Optionally, the write merge unit 55 provides the merged data to the network communication interface 25 to encode and prepare a response or return data packet for transmission on the chiplet network 85.

When the request data is for a built-in atomic operator, the built-in atomic unit 65 receives the request and reads data, either from the write merge unit 55 or directly from the off-die memory controller 20. The atomic operator is performed, and using the write merge unit 55, the resulting data is written to the cache 10, or provided to the network communication interface 25 to encode and prepare a response or return data packet for transmission on the chiplet network 85.

The built-in atomic unit 65 handles predefined atomic operators such as fetch-and-increment or compare-and-swap. In an example, these operations perform a simple read-modify-write operation to a single memory location of 32-bytes or less in size. Atomic memory operations are initiated from a request packet transmitted over the chiplet network 85. The request packet has a physical address, atomic operator type, operand size, and optionally up to 32-bytes of data. The atomic operator performs the read-modify-write to a cache memory line of the cache 10, filling the cache memory if necessary. The atomic operator response can be a simple completion response, or a response with up to 32-bytes of data. Example atomic memory operators include fetch-and-AND, fetch-and-OR, fetch-and-XOR, fetch-and-add, fetch-and-subtract, fetch-and-increment, fetch-and-decrement, fetch-and-minimum, fetch-and-maximum, fetch-and-swap, and compare-and-swap. In various example embodiments 32-bit and 64-bit operations are supported, along with operations on 16 or 32 bytes of data. Methods disclosed herein are also compatible with hardware supporting larger or smaller operations and more or less data.

Built-in atomic operators can also involve requests for a “standard” atomic operator on the requested data, such as comparatively simple, single cycle, integer atomics—such as fetch-and-increment or compare-and-swap—which will occur with the same throughput as a regular memory read or write operation not involving an atomic operator. For these operations, the cache controller 15 can generally reserve a cache line in the cache 10 by setting a hazard bit (in hardware), so that the cache line cannot be read by another process while it is in transition. The data is obtained from either the off-die memory 75 or the cache 10, and is provided to the built-in atomic unit 65 to perform the requested atomic operator. Following the atomic operator, in addition to providing the resulting data to the packet encoder 40 to encode outgoing data packets for transmission on the chiplet network 85, the built-in atomic unit 65 provides the resulting data to the write merge unit 55, which will also write the resulting data to the cache 10. Following the writing of the resulting data to the cache 10, any corresponding hazard bit which was set will be cleared by the memory hazard unit 60.

The PAU 70 enables high performance (high throughput and low latency) for programmable atomic operators (also referred to as “custom atomic transactions” or “custom atomic operators”), comparable to the performance of built-in atomic operators. Rather than executing multiple memory accesses, in response to an atomic operator request designating a programmable atomic operator and a memory address, circuitry in the memory controller chiplet 05 transfers the atomic operator request to PAU 70 and sets a hazard bit stored in a memory hazard register corresponding to the memory address of the memory line used in the atomic operator, to ensure that no other operation (read, write, or atomic) is performed on that memory line, which hazard bit is then cleared upon completion of the atomic operator. Additional, direct data paths provided for the PAU 70 executing the programmable atomic operators allow for additional write operations without any limitations imposed by the bandwidth of the communication networks and without increasing any congestion of the communication networks.

The PAU 70 includes a multi-threaded processor, for example, such as a RISC-V ISA based multi-threaded processor, having one or more processor cores, and further having an extended instruction set for executing programmable atomic operators. When provided with the extended instruction set for executing programmable atomic operators, the PAU 70 can be embodied as one or more hybrid threading processors. In some example embodiments, the PAU 70 provides barrel-style, round-robin instantaneous thread switching to maintain a high instruction-per-clock rate.

Programmable atomic operators can be performed by the PAU 70 involving requests for a programmable atomic operator on the requested data. A user can prepare programming code to provide such programmable atomic operators. For example, the programmable atomic operators can be comparatively simple, multi-cycle operations such as floating-point addition, or comparatively complex, multi-instruction operations such as a Bloom filter insert. The programmable atomic operators can be the same as or different than the predetermined atomic operators, insofar as they are defined by the user rather than a system vendor. For these operations, the cache controller 15 can reserve a cache line in the cache 10, by setting a hazard bit (in hardware), so that cache line cannot be read by another process while it is in transition. The data is obtained from either the cache 10 or the off-die memories 75 or 80, and is provided to the PAU 70 to perform the requested programmable atomic operator. Following the atomic operator, the PAU 70 will provide the resulting data to the network communication interface 25 to directly encode outgoing data packets having the resulting data for transmission on the chiplet network 85. In addition, the PAU 70 will provide the resulting data to the cache controller 15, which will also write the resulting data to the cache 10. Following the writing of the resulting data to the cache 10, any corresponding hazard bit which was set will be cleared by the cache control circuit 15.

In selected examples, the approach taken for programmable atomic operators is to provide multiple, generic, custom atomic request types that can be sent through the chiplet network 85 to the memory controller chiplet 05 from an originating source such as a processor or other system component. The cache controllers 15 or off-die memory controller 20 identify the request as a custom atomic and forward the request to the PAU 70. In a representative embodiment, the PAU 70: (1) is a programmable processing element capable of efficiently performing a user defined atomic operator; (2) can perform load and stores to memory, arithmetic and logical operations and control flow decisions; and (3) leverages the RISC-V ISA with a set of new, specialized instructions to facilitate interacting with such controllers 15, 20 to atomically perform the user-defined operation. In desirable examples, the RISC-V ISA contains a full set of instructions that support high level language operators and data types. The PAU 70 can leverage the RISC-V ISA, but will commonly support a more limited set of instructions and limited register file size to reduce the die size of the unit when included within the memory controller chiplet 05.

As mentioned above, prior to the writing of the read data to the cache 10, the set hazard bit for the reserved cache line is to be cleared, by the memory hazard clear unit 60. Accordingly, when the request and read data is received by the write merge unit 55, a reset or clear signal can be transmitted by the memory hazard clear unit 60 to the cache 10 to reset the set memory hazard bit for the reserved cache line. Also, resetting this hazard bit will also release a pending read or write request involving the designated (or reserved) cache line, providing the pending read or write request to an inbound request multiplexer for selection and processing.

FIG. 3 illustrates components in an example of a programmable atomic unit 300 (PAU), such as those noted above with respect to FIG (e.g., in the memory controller 40) and FIG. (e.g., PAU 70), according to an embodiment. As illustrated, the PAU 300 includes a processor 305, local memory 310 (e.g., SRAM), and a controller 315 for the local memory 310.

In an example, the processor 305 is a pipelined such that multiple stages of different instructions are executed together per clock cycle. The processor 305 is also a barrel-multithreaded processor, with circuitry to switch between different register files (e.g., sets of registers containing current processing state) upon each clock cycle of the processor 305. This enables efficient context switching between currently executing threads. In an example, the processor 305 supports eight threads, resulting in eight register files. In an example, some or all of the register files are not integrated into the processor 305, but rather reside in the local memory 310 (registers 320). This reduces circuit complexity in the processor 305 by eliminating the traditional flip-flops used for these registers 320.

The local memory 310 can also house a cache 330 and instructions 325 for atomic operators. The atomic instructions 325 comprise sets of instructions to support the various application-loaded atomic operators. When an atomic operator is requested—by the application chiplet 25, for example—a set of instructions corresponding to the atomic operator are executed by the processor 305. In an example, the atomic instructions 325 are partitioned to establish the sets of instructions. In this example, the specific programmable atomic operator being requested by a requesting process can identify the programmable atomic operator by the partition number. The partition number can be established when the programmable atomic operator is registered with (e.g., loaded onto) the PAU 300. Additional metadata for the programmable atomic instructions 325 can also be stored in the local memory 310, such as the partition tables.

Atomic operators manipulate the cache 330, which is generally synchronized (e.g., flushed) when a thread for an atomic operator completes. Thus aside from initial loading from the external memory such as the off-die memory 75 or 80, latency is reduced for most memory operations during execution of a programmable atomic operator thread.

As noted above, a pipelined processor, such as the processor 305, can experience an issue when an executing thread attempts to issue a memory request if an underlying hazard condition would prevent such a request. Here, the memory request is to retrieve data from the memory controller, whether it be from a cache on the memory controller or off-die memory. To resolve this issue, the processor 305 is configured to deny the memory request for a thread. Generally, the processor 305 includes circuitry to enable one or more thread rescheduling points in the pipeline. Here, the denial occurs at a point in the pipeline that is beyond (e.g., after) these thread rescheduling points. In an example, the hazard occurred beyond the rescheduling point. Here, a preceding instruction in the thread created the hazard after the memory request instruction passed the last thread rescheduling point prior to the pipeline stage in which the memory request could be made.

In an example, to deny the memory request, the processor 305 is configured to determine (e.g., detect) that there is a hazard on memory indicated in the memory request. Here, hazard denotes any condition such that allowing (e.g., performing) the memory request will result in an inconsistent state for the thread. In an example, the hazard is an in-flight memory request. Here, whether or not the cache 330 includes data for the requested memory address, the presence of the in-flight memory request makes it uncertain what the data in the cache 330 at that address should be. Thus, the thread must wait for the in-flight memory request to be completed to operate on current data. Thus, the hazard is cleared when the memory request completes.

In an example, the hazard is a dirty cache line in the cache 330 for the requested memory address. Although the dirty cache line generally indicates that the data in the cache 330 is current and the memory controller version of this data is not, an issue can arise on thread instructions that do not operate from the cache 330. An example of such an instruction uses a built-in atomic operator, or other separate hardware block, of the memory controller. In the context of the memory controller 05, the built-in atomic operators 65 and separate from the PAU 70 and do not have access of the cache 300 inside the PAU 70. Thus, a memory request for a built-in atomic operator is processed from the cache 10 or the off-die memory 75 or 80. If the cache line in the cache 300 is dirty, then the built-in atomic operator will not be operating on the most current data until the cache 300 is flushed to synchronize the cache 10 and the off-die memories 75 and 80. This same situation could occur with other hardware blocks of the memory controller, such as cryptography block, encoder etc. Here, the hazard is cleared when the cache 330 is flushed.

The processor 305 is configured to place the thread into a memory response path of the processor. Although the memory request of the thread was not issued, the thread is largely handled as-if the memory request was made. This enables a smooth rescheduling of the thread without impacting other threads and without adding thread stalling circuitry to the various pipeline stages of the processor 305. Two issues can arise with this approach. First, because a memory request was not actually issued, the thread can skip some stages of the memory request and response path. Second because no memory response is coming, data will not be updated (e.g., there will be no register writeback) by the thread.

To address the first issue, the processor 305 can be configured to add the thread to a queue of threads awaiting insertion into the memory response path. Here, the processor 305 maintains the queue and pushes the thread onto the queue. In this way, the memory response path of the processor 305 can pop a next thread off the queue to place into the standard memory response path.

To address the second issue, the processor 305 is configured to provide an indicator that a register write-back will not occur for the thread. The indicator (e.g., flag) can be some bits added to the thread metadata in the queue, or elsewhere, to indicate that a write-back mechanism will not operate with respect to the thread in this instance. Thus, when the thread appears at a register file write block in the memory response path, no register fil write will occur.

In an example, to provide the indicator that a register write-back will not occur for the thread, the processor 305 is configured to create a memory response that includes the indicator. Here, the way in which the thread is inserted into the memory response path is through a memory response manufactured by the processor 305 rather than from the memory controller. Thus, to the memory response path, a standard memory response is being handled for the thread. However, the inclusion of the indicator means that a register or cache write block will not perform any actions. In an example, the processor 305 created memory response is placed into the queue to await insertion into the memory response path after earlier stages in the memory response path, such as decode from the memory controller, error correction, etc.

The processor 305 is configured to reschedule the thread with other threads in the memory response path. In an example, the thread is popped from the queue of threads awaiting insertion into the memory response path. In an example, the indicator is processed by the memory response path to move the thread from the queue of threads awaiting a response from memory to a memory response reschedule queue in the memory response path. Here, the indicator, or other metadata, is used by the memory response path to determine a correct insertion time for the thread amongst other pending threads.

FIG. 4 illustrates a processing flow through processor components to reschedule a failed memory request, according to an embodiment. Elements 405-420 illustrate processor pipeline stages from checking the cache 405, to error correction 410, to cache hit detection 415, to issuing a memory request 420. In a successful memory request by a thread, the process would proceed to wait for a response from the memory controller 430 and merge 440 results from the memory controller and the cache 425 (after undergoing error correction 435).

Using the devices and techniques discussed herein, the dashed line indicates the rescheduling path (e.g., bypass circuitry) for a thread that is not allowed to make the intended memory request. Here, the thread (e.g., a manufacture memory response for the thread) is placed in the bypass queue 450. Entries in the queue 450 are popped into register file write and scheduler circuitry 445, along with standard memory responses, to be rescheduled. However, as noted above, the indicator in the thread will prevent the register file writer from executing any such write. By using such a counterfeit memory response, the traditional memory response path can be used to reschedule the thread

This technique greatly simplifies cases where threads in a barrel-multithreaded processor are unable to issue memory requests late in the pipeline. Stalling to wait for the hazard to clear would add latency to all allocated threads and a logic cost (e.g., additional circuitry) to all stages of the pipeline to handle the stall. By enabling the thread to halt and reschedule via the existing successful memory response path, hazards can be cleared without costing any additional cycles and the thread is rescheduled with no penalty to other threads. Additional details and examples are provided below.

FIG. 5 is a flow chart of an example of a method 500 for rescheduling failed memory request in a processor, according to an embodiment. Operations of the method 500 are performed by computer hardware such as that described with respect to FIG. (e.g., memory controller chiplet 40), FIG., or FIG. (e.g., processing circuitry).

At operation 505, a memory request is denied for a thread. Here, the denial occurs at a point in a thread execution pipeline of the processor that is beyond a thread rescheduling point. In an example, denying the memory request for the thread includes determining that there is a hazard on memory indicated in the memory request. In an example, the hazard is a dirty cache line of the processor. In an example the memory request is for a built-in atomic operator of a memory controller for the memory indicated in the memory request. In an example, the hazard occurred beyond the rescheduling point.

At operation 510, the thread is placed into a memory response path of the processor. In an example, placing the thread into the memory response path includes adding the thread to a queue of threads awaiting insertion into the memory response path.

At operation 515, an indicator that a register write-back will not occur for the thread is provided. In an example, providing the indicator that a register write-back will not occur for the thread includes creating a memory response that includes the indicator.

At operation 520 the thread is rescheduled with other threads in the memory response path. In an example, the indicator is processed by the memory response path to move the thread from the queue of threads awaiting a response from memory into the general thread scheduler queue by way of the memory response path.

FIG. illustrates a block diagram of an example machine 00 with which, in which, or by which any one or more of the techniques (e.g., methodologies) discussed herein can be implemented. Examples, as described herein, can include or can operate by, logic or a number of components, or mechanisms in the machine 00. Circuitry (e.g., processing circuitry) is a collection of circuits implemented in tangible entities of the machine 00 that include hardware (e.g., simple circuits, gates, logic, etc.). Circuitry membership can be flexible over time. Circuitries include members that can, alone or in combination, perform specified operations when operating. In an example, hardware of the circuitry can be immutably designed to carry out a specific operation (e.g., hardwired). In an example, the hardware of the circuitry can include variably connected physical components (e.g., execution units transistors, simple circuits, etc.) including a machine readable medium physically modified (e.g., magnetically, electrically, moveable placement of invariant massed particles, etc.) to encode instructions of the specific operation. In connecting the physical components, the underlying electrical properties of a hardware constituent are changed, for example, from an insulator to a conductor or vice versa. The instructions enable embedded hardware (e.g., the execution units or a loading mechanism) to create members of the circuitry in hardware via the variable connections to carry out portions of the specific operation when in operation. Accordingly, in an example, the machine-readable medium elements are part of the circuitry or are communicatively coupled to the other components of the circuitry when the device is operating. In an example, any of the physical components can be used in more than one member of more than one circuitry. For example, under operation, execution units can be used in a first circuit of a first circuitry at one point in time and reused by a second circuit in the first circuitry, or by a third circuit in a second circuitry at a different time. Additional examples of these components with respect to the machine 00 follow.

In alternative embodiments, the machine 00 can operate as a standalone device or can be connected (e.g., networked) to other machines. In a networked deployment, the machine 00 can operate in the capacity of a server machine, a client machine, or both in server-client network environments. In an example, the machine 00 can act as a peer machine in peer-to-peer (P2P) (or other distributed) network environment. The machine 00 can be a personal computer (PC), a tablet PC, a set-top box (STB), a personal digital assistant (PDA), a mobile telephone, a web appliance, a network router, switch or bridge, or any machine capable of executing instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein, such as cloud computing, software as a service (SaaS), other computer cluster configurations.

The machine (e.g., computer system) 00 can include a hardware processor 02 (e.g., a central processing unit (CPU), a graphics processing unit (GPU), a hardware processor core, or any combination thereof), a main memory 04, a static memory (e.g., memory or storage for firmware, microcode, a basic-input-output (BIOS), unified extensible firmware interface (UEFI), etc.) 06, and mass storage 08 (e.g., hard drives, tape drives, flash storage, or other block devices) some or all of which can communicate with each other via an interlink (e.g., bus) 30. The machine 00 can further include a display unit 10, an alphanumeric input device 12 (e.g., a keyboard), and a user interface (UI) navigation device 14 (e.g., a mouse). In an example, the display unit 10, input device 12 and UI navigation device 14 can be a touch screen display. The machine 00 can additionally include a storage device (e.g., drive unit) 08, a signal generation device 18 (e.g., a speaker), a network interface device 20, and one or more sensors 16, such as a global positioning system (GPS) sensor, compass, accelerometer, or other sensor. The machine 00 can include an output controller 28, such as a serial (e.g., universal serial bus (USB), parallel, or other wired or wireless (e.g., infrared (IR), near field communication (NFC), etc.) connection to communicate or control one or more peripheral devices (e.g., a printer card reader, etc.).

Registers of the processor 02, the main memory 04, the static memory 06, or the mass storage 08 can be, or include, a machine readable medium 22 on which is stored one or more sets of data structures or instructions 24 (e.g., software) embodying or utilized by any one or more of the techniques or functions described herein. The instructions 24 can also reside, completely or at least partially, within any of registers of the processor 02, the main memory 04, the static memory 06, or the mass storage 08 during execution thereof by the machine 00. In an example, one or any combination of the hardware processor 02, the main memory 04, the static memory 06, or the mass storage 08 can constitute the machine readable media 22. While the machine readable medium 22 is illustrated as a single medium, the term “machine readable medium” can include a single medium or multiple media (e.g., a centralized or distributed database, or associated caches and servers) configured to store the one or more instructions 24.

The term “machine readable medium” can include any medium that is capable of storing, encoding, or carrying instructions for execution by the machine 00 and that cause the machine 00 to perform any one or more of the techniques of the present disclosure, or that is capable of storing, encoding or carrying data structures used by or associated with such instructions. Non-limiting machine-readable medium examples can include solid-state memories, optical media, magnetic media, and signals (e.g., radio frequency signals, other photon-based signals, sound signals, etc.). In an example, a non-transitory machine-readable medium comprises a machine-readable medium with a plurality of particles having invariant (e.g., rest) mass, and thus are compositions of matter. Accordingly, non-transitory machine-readable media are machine readable media that do not include transitory propagating signals. Specific examples of non-transitory machine readable media can include: non-volatile memory, such as semiconductor memory devices (e.g., electrically programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM)) and flash memory devices; magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks.

In an example, information stored or otherwise provided on the machine readable medium 22 can be representative of the instructions 24, such as instructions 24 themselves or a format from which the instructions 24 can be derived. This format from which the instructions 24 can be derived can include source code, encoded instructions (e.g., in compressed or encrypted form), packaged instructions (e.g., split into multiple packages), or the like. The information representative of the instructions 24 in the machine readable medium 22 can be processed by processing circuitry into the instructions to implement any of the operations discussed herein. For example, deriving the instructions 24 from the information (e.g., processing by the processing circuitry) can include: compiling (e.g., from source code object code, etc.) interpreting, loading organizing (e.g., dynamically or statically linking), encoding decoding, encrypting, unencrypting, packaging, unpackaging, or otherwise manipulating the information into the instructions 24.

In an example, the derivation of the instructions 24 can include assembly, compilation, or interpretation of the information (e.g., by the processing circuitry) to create the instructions 24 from some intermediate or preprocessed format provided by the machine readable medium 22. The information, when provided in multiple parts, can be combined, unpacked, and modified to create the instructions 24. For example, the information can be in multiple compressed source code packages (or object code, or binary executable code etc.) on one or several remote servers. The source code packages can be encrypted when in transit over a network and decrypted, uncompressed, assembled (e.g., linked) if necessary, and compiled or interpreted (e.g., into a library, stand-alone executable etc.) at a local machine, and executed by the local machine.

The instructions 24 can be further transmitted or received over a communications network 26 using a transmission medium via the network interface device 20 utilizing any one of a number of transfer protocols (e.g., frame relay, internee protocol (IP), transmission control protocol (TCP), user datagram protocol (UDP), hypertext transfer protocol (HTTP), etc.). Example communication networks can include a local area network (LAN), a wide area network (WAN), a packet data network (e.g., the Internet), mobile telephone networks (e.g., cellular networks), plain old telephone (POTS) networks, and wireless data networks (e.g., Institute of Electrical and Electronics Engineers (IEEE) 802.11 family of standards known as Wi-Fi®, IEEE 802.16 family of standards known as WiMax®), IEEE 802.15.4 family of standards, peer-to-peer (P2P) networks, among others. In an example, the network interface device 20 can include one or more physical jacks (e.g., Ethernet, coaxial, or phone jacks) or one or more antennas to connect to the communications network 26. In an example, the network interface device 20 can include a plurality of antennas to wirelessly communicate using at least one of single-input multiple-output (SIMO), multiple-input multiple-output (MIMO), or multiple-input single-output (MISO) techniques. The term “transmission medium” shall be taken to include any intangible medium that is capable of storing, encoding or carrying instructions for execution by the machine 00, and includes digital or analog communications signals or other intangible medium to facilitate communication of such software. A transmission medium is a machine readable medium. To better illustrate the methods and apparatuses described herein, a non-limiting set of Example embodiments are set forth below as numerically identified Examples.

Example 1 is a processor comprising: execution pipeline configured to deny a memory request for a thread at a point in the execution pipeline beyond a thread rescheduling point; bypass circuitry configured to: place the thread into a memory response path of the processor; and provide an indicator that a register write-back will not occur for the thread; and scheduler circuitry configured to reschedule the thread with other threads in the memory response path.

In Example 2, the subject matter of Example 1, wherein, to deny the memory request for the thread, the execution pipeline is configured to determine that there is a hazard on memory indicated in the memory request.

In Example 3, the subject matter of Example 2, wherein the hazard is a dirty cache line of the processor.

In Example 4, the subject matter of Example 3, wherein the memory request is for a built-in atomic operator of a memory controller for the memory indicated in the memory request.

In Example 5, the subject matter of any of Examples 2-4, wherein the hazard occurred beyond the rescheduling point.

In Example 6, the subject matter of any of Examples 1-5, comprising a queue of threads awaiting insertion into the memory response path, wherein, to place the thread into the memory response path the bypass circuitry is configured to add the thread to the queue of threads awaiting insertion into the memory response path.

In Example 7, the subject matter of Example 6, wherein, provide the indicator that a register write-back will not occur for the thread, the bypass circuitry is configured to create a memory response that includes the indicator.

In Example 8, the subject matter of Example 7, wherein the indicator is processed by the memory response path to move the thread from the queue of threads awaiting insertion into the memory response path into the general thread scheduler queue by way of the memory response path.

Example 9 is a method comprising: denying, at a point in a thread execution pipeline of a processor, a memory request for a thread, the point in the thread execution pipeline beyond a thread rescheduling point; placing the thread into a memory response path of the processor; providing an indicator that a register write-back will not occur for the thread; and rescheduling the thread with other threads in the memory response path.

In Example 10, the subject matter of Example 9, wherein denying the memory request for the thread includes determining that there is a hazard on memory indicated in the memory request.

In Example 11, the subject matter of Example 10, wherein the hazard is a dirty cache line of the processor.

In Example 12, the subject matter of Example 11, wherein the memory request is for a built-in atomic operator of a memory controller for the memory indicated in the memory request.

In Example 13, the subject matter of any of Examples 10-12, wherein the hazard occurred beyond the rescheduling point.

In Example 14, the subject matter of any of Examples 9-13, wherein placing the thread into the memory response path includes adding the thread to a queue of threads awaiting insertion into the memory response path.

In Example 15, the subject matter of Example 14, wherein providing the indicator that a register write-back will not occur for the thread includes creating a memory response that includes the indicator.

In Example 16, the subject matter of Example 15, wherein the indicator is processed by the memory response path to move the thread from the queue of threads awaiting insertion into the memory response path into the general thread scheduler queue by way of the memory response path.

Example 17 is a machine-readable medium including instructions that, when executed by circuitry of a processor, cause the processor to perform operations comprising: denying, at a point in a thread execution pipeline of the processor, a memory request for a thread, the point in the thread execution pipeline beyond a thread rescheduling point; placing the thread into a memory response path of the processor; providing an indicator that a register write-back will not occur for the thread; and rescheduling the thread with other threads in the memory response path.

In Example 18, the subject matter of Example 17, wherein denying the memory request for the thread includes determining that there is a hazard on memory indicated in the memory request.

In Example 19, the subject matter of Example 18, wherein the hazard is a dirty cache line of the processor.

In Example 20, the subject matter of Example 19, wherein the memory request is for a built-in atomic operator of a memory controller for the memory indicated in the memory request.

In Example 21, the subject matter of any of Examples 18-20, wherein the hazard occurred beyond the rescheduling point.

In Example 22, the subject matter of any of Examples 17-21, wherein placing the thread into the memory response path includes adding the thread to a queue of threads awaiting insertion into the memory response path.

In Example 23, the subject matter of Example 22, wherein providing the indicator that a register write-back will not occur for the thread includes creating a memory response that includes the indicator.

In Example 24, the subject matter of Example 23, wherein the indicator is processed by the memory response path to move the thread from the queue of threads awaiting insertion into the memory response path into the general thread scheduler queue by way of the memory response path.

Example 25 is a system comprising: means for denying, at a point in a thread execution pipeline of a processor, a memory request for a thread, the point in the thread execution pipeline beyond a thread rescheduling point; means for placing the thread into a memory response path of the processor; means for providing an indicator that a register write-back will not occur for the thread; and means for rescheduling the thread with other threads in the memory response path.

In Example 26, the subject matter of Example 25, wherein the means for denying the memory request for the thread include means for determining that there is a hazard on memory indicated in the memory request.

In Example 27, the subject matter of Example 26, wherein the hazard is a dirty cache line of the processor.

In Example 28, the subject matter of Example 27, wherein the memory request is for a built-in atomic operator of a memory controller for the memory indicated in the memory request.

In Example 29, the subject matter of any of Examples 26-28, wherein the hazard occurred beyond the rescheduling point.

In Example 30, the subject matter of any of Examples 25-29, wherein the means for placing the thread into the memory response path include means for adding the thread to a queue of threads awaiting insertion into the memory response path.

In Example 31, the subject matter of Example 30, wherein the means for providing the indicator that a register write-back will not occur for the thread include means for creating a memory response that includes the indicator.

In Example 32, the subject matter of Example 31, wherein the indicator is processed by the memory response path to move the thread from the queue of threads awaiting insertion into the memory response path into the general thread scheduler queue by way of the memory response path.

Example 33 is at least one machine-readable medium including instructions that, when executed by processing circuitry, cause the processing circuitry to perform operations to implement of any of Examples 1-32.

Example 34 is an apparatus comprising means to implement of any of Examples 1-32.

Example 35 is a system to implement of any of Examples 1-32.

Example 36 is a method to implement of any of Examples 1-32.

The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples”. Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein.

In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” can include “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In the appended claims the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein”. Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.

The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) can be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features can be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter can lie in less than all features of a particular disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 

What is claimed is:
 1. An apparatus comprising: an execution pipeline configured to deny a memory request for a thread at a point in the execution pipeline beyond a thread rescheduling point; bypass circuitry configured to: place the thread into a memory response path of the apparatus; and provide an indicator that a register write-back will not occur for the thread; and scheduler circuitry configured to reschedule the thread with other threads in the memory response path.
 2. The apparatus of claim 1, wherein, to deny the memory request for the thread, the execution pipeline is configured to determine that there is a hazard on memory indicated in the memory request.
 3. The apparatus of claim 2, wherein the hazard is a dirty cache line of the apparatus.
 4. The apparatus of claim 3, wherein the memory request is for a built-in atomic operator of a memory controller for the memory indicated in the memory request.
 5. The apparatus of claim 2, wherein the hazard occurred beyond the rescheduling point.
 6. The apparatus of claim 1, comprising a queue of threads awaiting insertion into the memory response path, wherein, to place the thread into the memory response path the bypass circuitry is configured to add the thread to the queue of threads awaiting insertion into the memory response path.
 7. The apparatus of claim 6, wherein, provide the indicator that a register write-back will not occur for the thread, the bypass circuitry is configured to create a memory response that includes the indicator.
 8. The apparatus of claim 7, wherein the indicator is processed by the memory response path to move the thread from the queue of threads awaiting insertion into the memory response path into the general thread scheduler queue by way of the memory response path.
 9. A method comprising: denying, at a point in a thread execution pipeline of a processor, a memory request for a thread, the point in the thread execution pipeline beyond a thread rescheduling point; placing the thread into a memory response path of the processor; providing an indicator that a register write-back will not occur for the thread; and rescheduling the thread with other threads in the memory response path.
 10. The method of claim 9, wherein denying the memory request for the thread includes determining that there is a hazard on memory indicated in the memory request.
 11. The method of claim 10, wherein the hazard is a dirty cache line of the processor.
 12. The method of claim 11, wherein the memory request is for a built-in atomic operator of a memory controller for the memory indicated in the memory request.
 13. The method of claim 10, wherein the hazard occurred beyond the rescheduling point.
 14. The method of claim 9, wherein placing the thread into the memory response path includes adding the thread to a queue of threads awaiting insertion into the memory response path.
 15. The method of claim 14, wherein providing the indicator that a register write-back will not occur for the thread includes creating a memory response that includes the indicator.
 16. The method of claim 15, wherein the indicator is processed by the memory response path to move the thread from the queue of threads awaiting insertion into the memory response path into the general thread scheduler queue by way of the memory response path.
 17. A machine-readable medium including instructions that, when executed by circuitry of a processor, cause the processor to perform operations comprising: denying, at a point in a thread execution pipeline of the processor, a memory request for a thread, the point in the thread execution pipeline beyond a thread rescheduling point; placing the thread into a memory response path of the processor; providing an indicator that a register write-back will not occur for the thread; and rescheduling the thread with other threads in the memory response path.
 18. The machine-readable medium of claim 17, wherein denying the memory request for the thread includes determining that there is a hazard on memory indicated in the memory request.
 19. The machine-readable medium of claim 18, wherein the hazard is a dirty cache line of the processor.
 20. The machine-readable medium of claim 19, wherein the memory request is for a built-in atomic operator of a memory controller for the memory indicated in the memory request.
 21. The machine-readable medium of claim 18, wherein the hazard occurred beyond the rescheduling point.
 22. The machine-readable medium of claim 17, wherein placing the thread into the memory response path includes adding the thread to a queue of threads awaiting insertion into the memory response path.
 23. The machine-readable medium of claim 22, wherein providing the indicator that a register write-back will not occur for the thread includes creating a memory response that includes the indicator.
 24. The machine-readable medium of claim 23, wherein the indicator is processed by the memory response path to move the thread from the queue of threads awaiting insertion into the memory response path into the general thread scheduler queue by way of the memory response path. 